src/test/ui/run-pass/issues/issue-31299.rs

   1 // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 // run-pass
  12 // Regression test for #31299. This was generating an overflow error
  13 // because of eager normalization:
  14 //
  15 // proving `M: Sized` requires
  16 // - proving `PtrBack<Vec<M>>: Sized` requires
  17 //   - normalizing `Vec<<Vec<M> as Front>::Back>>: Sized` requires
  18 //     - proving `Vec<M>: Front` requires
  19 //       - `M: Sized` <-- cycle!
  20 //
  21 // If we skip the normalization step, though, everything goes fine.
  22 //
  23 // This could be fixed by implementing lazy normalization everywhere.
  24 //
  25 // However, we want this to work before then. For that, when checking
  26 // whether a type is Sized we only check that the tails are Sized. As
  27 // PtrBack does not have a tail, we don't need to normalize anything
  28 // and this compiles
  29
  30 trait Front {
  31     type Back;
  32 }
  33
  34 impl<T> Front for Vec<T> {
  35     type Back = Vec<T>;
  36 }
  37
  38 struct PtrBack<T: Front>(Vec<T::Back>);
  39
  40 struct M(PtrBack<Vec<M>>);
  41
  42 fn main() {
  43     std::mem::size_of::<M>();
  44 }